Aqueous polymer dispersions, based on copolymers of vinyl aromatics and butadiene, method for their production and their use as sizing agents for paper

ABSTRACT

Aqueous polymer dispersions which are obtainable by free radical copolymerization of
     (a) from 0.1 to 99.9% by weight of styrene and/or methylstyrene,   (b) 0.1-99.9% by weight of 1,3-butadiene and/or isoprene and   (c) from 0 to 40% by weight of other ethylenically unsaturated copolymerizable monomers, the sum of the monomers (a), (b) and (c) always being 100,
 
in the presence of from 10 to 40% by weight, based on the monomers used, of at least one degraded starch having a molecular weight Mn of from 500 to 40 000 and of water-soluble redox catalysts are prepared by free radical copolymerization of the monomers (a), (b) and, if required, (c) in an aqueous medium in the presence of a degraded starch having a molecular weight Mn of from 500 to 10 000 and redox initiators and are used as engine sizes and surface sizes for paper.

The present invention relates to aqueous polymer dispersions based oncopolymers of vinylaromatics and butadiene, processes for theirpreparation and their use as engine sizes and surface sizes for paper.

The use of aqueous polymer dispersions as sizes and as coating materialsfor paper is known. The use of starch and/or starch derivatives forstabilizing the polymer particles of such sizes is also described in theliterature. For example, JP-A-58/115,196 describes the preparation ofgraft copolymers which are obtainable by polymerizing 5-85% by weight ofstyrene and 2-50% by weight of (meth)acrylates in the presence ofwater-soluble polymers, such as starch. The aqueous dispersions whichcan thus be prepared are used as sizes for paper.

EP-A-0 257 412 discloses sizes for paper which are obtainable bypolymerizing a monomer mixture of 20-65% by weight of acrylonitrile,80-35% by weight of at least one acrylate and 0-10% by weight of otherethylenically unsaturated monomers in the aqueous phase in the presenceof a degraded starch having a reduced viscosity of 0.12-0.5 dl/g andredox initiators.

According to EP-A-0 276 770, sizes based on copolymers of acrylonitrileand acrylates are prepared by polymerizing the monomers in an aqueousmedium in the presence of a degraded starch having a reduced viscosityof from 0.04 to less than 0.12 dl/g and of redox catalysts.

EP-A-0 307 816 discloses a process for improving the printability ofpaper, an aqueous coating material comprising a pigment and a cationicaqueous polymer dispersion of a paper size and of a surface-activesubstance interfering with the formation of the surface size and/or of apolymeric dispersant being applied to one or both surfaces of the paper.

EP-A-0 735 065 describes the preparation of an amphoteric polymerdispersion by a two-stage polymerization. In the first stage,ethylenically unsaturated monomers and up to 30% by weight ofunsaturated carboxylic, sulfonic or phosphonic acids are polymerized inthe presence of enzymatically or hydrolytically degraded starch and/orstarch derivatives. In a second stage, further ethylenically unsaturatedmonomers and up to 35% by weight of cationic monomers are polymerized.

DE-A-198 53 489 relates to the use of aqueous styrene/butadienedispersions which are prepared by free radical polymerization of styreneand butadiene in the presence of protective colloids, such as polyvinylalcohol or water-soluble polysaccharides, in construction adhesiveformulations.

It is an object of the present invention to provide novel substanceswhich are suitable, for example, for the engine sizing and surfacesizing of paper.

We have found that this object is achieved, according to the invention,by aqueous polymer dispersions based on copolymers of vinylaromatics andbutadiene, which are obtainable by free radical copolymerization of

-   (a) from 0.1 to 99.9% by weight of styrene and/or methylstyrene,-   (b) 0.1-99.9% by weight of 1,3-butadiene and/or isoprene and-   (c) from 0 to 40% by weight of other ethylenically unsaturated    copolymerizable monomers, the sum of the monomers (a), (b) and (c)    always being 100,    in the presence of from 10 to 40% by weight, based on the monomers    used, of at least one degraded starch having a molecular weight Mn    of from 500 to 40 000 and of water-soluble redox catalysts.

The present invention also relates to a process for the preparation ofaqueous copolymer dispersions based on vinylaromatics and butadiene bycopolymerization of vinylaromatics and butadiene in an aqueous medium inthe presence of starch and water-soluble redox catalysts, wherein

-   (a) from 0.1 to 99.9% by weight of styrene and/or methylstyrene,-   (b) 0.1-99.9% by weight of 1,3-butadiene and/or isoprene and-   (c) from 0 to 40% by weight of other ethylenically unsaturated    copolymerizable monomers    are used in the copolymerization, the sum of the monomers (a), (b)    and (c) always being 100, and the copolymerization is carried out in    the presence of from 10 to 40% by weight, based on the monomers    used, of at least one degraded starch having a molecular weight Mn    of from 500 to 40 000.

In the novel process, a monomer mixture comprising

-   (a) from 50 to 99% by weight of styrene and/or methylstyrene,-   (b) from 1 to 50% by weight of butadiene and/or isoprene and-   (c) from 0 to 40% by weight of other ethylenically unsaturated    copolymerizable monomers    is preferably polymerized in an aqueous solution of an enzymatically    degraded natural starch with a redox catalyst comprising hydrogen    peroxide and heavy metal ions from the group consisting of the    cerium, manganese and iron(II) salts.

The present invention also relates to the use of the above-describedaqueous polymer dispersions as engine sizes and/or surface sizes forpaper, board and cardboard.

Suitable monomers of group (a) are, for example, styrene and substitutedstyrenes, such as α-methylstyrene, and mixtures of said monomers. Themonomer mixture used in the polymerization contains the vinylaromaticsof group (a) preferably in an amount of from 50.0 to 99.9, in particularfrom 75 to 99, % by weight. A vinylaromatic preferably used in thecopolymerization is styrene.

Suitable monomers of group (b) are 1,3-butadiene and substitutedbutadienes, such as 2-chlorobutadiene, or mixtures thereof. The monomersof group (b) are contained in the monomer mixture preferably in anamount of from 0.1 to 50, particularly preferably from 1 to 25, % byweight.

Suitable monomers of group (c) are anionic, cationic and/or nonionichydrophilic ethylenically unsaturated monomers. Examples of anionicmonomers are: acrylic acid, methacrylic acid, ethacrylic acid, crotonicacid, vinylacetic acid, itaconic acid, styrenesulfonic acid,acrylamido-2-methylpropanesulfonic acid, vinyl sulfonate,vinylphosphonic acid and/or maleic acid and their monoesters and thealkali metal and ammonium salts of these monomers. Mixtures of thesemonomers may also be used in the copolymerization.

The suitable monomers (c) are preferably water-soluble. They have, forexample, a solubility of at least 50 g/l of water at 20° C. Suitablemonomers (c) are, for example, acrylamide, methacrylamide,N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone,N-vinyloxazolidone, methylpolyglycol acrylates and methylpolyglycolmethacrylates.

Suitable cationic monomers are, for example,dialkylaminoalkylacrylamides, dialkylaminoalkyl acrylates and/ordialkylaminoalkylmethacrylamides and/or dialkylaminoalkyl methacrylates.Examples of these are esters of ethylenically unsaturated carboxylicacids with amino alcohols, such as dimethylaminoethyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl acrylate,diethylaminoethyl methacrylate, dimethylaminopropyl acrylate,dimethylaminopropyl methacrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate and diethylaminobutyl acrylate. The basicacrylates can be used in the form of the free bases, of the salts withmineral acids, such as hydrochloric acid, sulfuric acid and nitric acid,of the salts with organic acids, such as formic acid, acetic acid orpropionic acid, or of sulfonic acids or in quaternized form. Suitablequaternizing agents are, for example, dimethyl sulfate, diethyl sulfate,methyl chloride, ethyl chloride and benzyl chloride.

Further suitable comonomers are amides of ethylenically unsaturatedcarboxylic acids, such as acrylamide, methacrylamide and N-alkylmono-and diamides of monoethylenically unsaturated carboxylic acids havingalkyl radicals of 1 to 6 carbon atoms, e.g. N-methylacrylamide,N,N-dimethylacrylamide, N-methylmethacrylamide, N-ethylacrylamide,N-propylacrylamide and tert-butylacrylamide, and basic(meth)acrylamides, such as dimethylaminoethylacrylamide,dimethylaminoethylmethacrylamide, diethylaminoethylacrylamide anddiethylaminoethylmethacrylamide, dimethylaminopropylacrylamide,diethylaminopropylacrylamide, dimethylaminopropylmethacrylamide anddiethylaminopropylmethacrylamide.

Further suitable comonomers (c) are N-vinylimidazole and substitutedN-vinylimidazoles, such as N-vinyl-2-methylimidazole,N-vinyl-4-methylimidazole, N-vinyl-5-methylimidazole andN-vinyl-2-ethylimidazole, and N-vinylimidazolines, such asN-vinylimidazoline, N-vinyl-2-methylimidazoline andN-vinyl-2-ethylimidazoline. N-Vinylimidazoles and N-vinylimidazolinesare used not only in the form of the free bases but also in a formneutralized with mineral acids or organic acids or in quaternized form,the quaternization preferably being carried out with dimethyl sulfate,diethyl sulfate, methyl chloride or benzyl chloride.

The following are furthermore suitable as comonomers:

-   N-trimethylammoniumethylacrylamide chloride,-   N-trimethylammoniumethylmethacrylamide chloride,-   N-trimethylammoniumethyl methacrylate chloride,-   N-trimethylammoniumethyl acrylate chloride,-   trimethylammoniumethylacrylamide methosulfate,-   trimethylammoniumethylmethacrylamide methosulfate,-   N-ethyldimethylammoniummethylacrylamide ethosulfate,-   N-ethyldimethylammoniumethylmethacrylamide ethosulfate,-   trimethylammoniumpropylacrylamide chloride,-   trimethylammoniumpropylmethacrylamide,-   trimethylammoniumpropylacrylamide methosulfate,-   trimethylammoniumpropylmethacrylamide methosulfate and-   N-ethyldimethylammoniumpropylacrylamide ethosulfate.

Preferably used monomers of group (c) are acrylic acid, methacrylicacid, maleic acid, N-vinylformamide, acrylates and methacrylates andvinyl acetate.

For example, natural starches, such as potato, wheat, corn, rice ortapioca starch, are suitable as starch, potato starch being preferred.Starches containing at least 80% of amylopectin are preferred.Chemically modified starches, such as hydroxethyl- orhydroxypropyl-starches, or starches containing anionic groups, e.g.phosphate starch, or cationic starches which have quaternary ammoniumgroups may also be used.

The starch to be used according to the invention is obtained bysubjecting said starch types to oxidative, thermal, acidic or enzymaticdegradation. However, the starch can also be subjected to a combineddegradation, for example a hydrolytic and an oxidative degradation. Inorder to establish the desired molecular weight of the starch, it ispreferably enzymatically degraded. Starch degradation with termamyl, asusually carried out when improving the solubility properties of thestarch, and a further degradation, for example with hydrogen peroxide,which can be carried out, for example, shortly before the subsequentgraft copolymerization, are particularly preferred. In this case,hydrogen peroxide (calculated as 100%) in concentrations of, forexample, from 0.3 to 5.0% by weight, based on starch used, is used. Theamount of hydrogen peroxide depends on the molecular weight to which thestarch is to be degraded in each case.

The starches degraded in this manner have an average molecular weight Mnof from 500 to 40 000, preferably from 500 to 10 000, with the resultthat, on the one hand, good dispersing of the emulsion polymers isensured and, on the other hand, precipitation of the polymerizationbatch is avoided. The average molecular weight of the degraded starchcan readily be determined with the aid of known gel chromatographicanalysis methods after calibration, for example with dextran standards.Viscosimetric methods, as described, for example, in Methods inCarbohydrate Chemistry, Volume IV, Academic Press New York andFrankfurt, 1964, page 127, are also suitable for the characterization.The intrinsic viscosity of the degraded starches which is determined inthis manner is preferably from 0.05 to 0.12 dl/g.

The polymerization of the monomers (a), (b) and, if required, (c) iscarried out, as a rule, by adding both the monomers, either individuallyor as a mixture, and the redox initiator suitable for initiating thepolymerization to the aqueous solution of degraded starch.

In order to increase the dispersing effect, low molecular weight anionicor nonionic emulsifiers, such as sodium alkanesulfonate, sodiumdodecylsulfate, sodium dodecylbenzenesulfonate, sulfosuccinic esters,fatty alcohol polyglycol ethers, alkylaryl polyglycol ethers, etc., canbe added to the polymerization batch. As a rule, however, suchemulsifiers result in a poorer sizing effect of the polymer dispersionsand generally lead to undesirable frothing during the handling of thedispersions. The polymerization is therefore preferably carried out inthe absence of an emulsifier.

However, polymeric anionic emulsifiers which contain sulfo groups, forexample based on maleic anhydride copolymers, are suitable.

The polymerization is usually carried out in the absence of oxygen,preferably in an inert gas atmosphere, for example under nitrogen.During the polymerization, thorough mixing with the aid of a suitablestirrer should be ensured.

The polymerization can be carried out both by the feed method and by abatch method, for example at from 30 to 100° C., preferably from 70 to95° C.

In the feed method, which is preferable for obtaining a finely divideddispersion, the monomers and the free radical initiator are meteredsimultaneously into the starch solution in a stirred kettle. In order toobtain particular effects, a nonuniform or staggered addition ofindividual components may also be effected. The reaction times are, forexample, from 0.5 to 10, preferably from 0.75 to 4, hours.

Graft-linking water-soluble redox systems are suitable for initiatingthe polymerization. For example, conventional water-soluble initiators,such as potassium peroxodisulfate, sodium peroxodisulfate, ammoniumperoxodisulfate, hydrogen peroxide, etc., can be used together with atleast one conventional reducing agent, such as sodium sulfite, sodiumdisulfite, sodium hydrogen sulfite, sodium dithionite, ascorbic acid orthe sodium salt of hydroxymethanesulfonic acid, etc., as a redox system.Such redox systems lead in most cases to coarser-particled dispersions.

Particularly suitable redox catalysts having high grafting activity arewater-soluble initiator systems, such as redox systems comprisinghydrogen peroxide and heavy metal ions, such as cerium, manganese oriron(II) salts, as described, for example, in Houben-Weyl, Methoden derorganischen Chemie 4th edition, Volume E20, page 2168. The redox systemcomprising hydrogen peroxide and an iron(II) salt, such as iron(II)sulfate, is particularly suitable and gives finely divided dispersionshaving a high grafting yield. The grafting yield is understood asmeaning the proportion of the polymer which is chemically coupled to thestarch after the end of the polymerization. The grafting yield should beas high as possible in order to obtain finely divided and highlyeffective dispersions.

The polymerization is usually carried out by adding the heavy metal saltof the redox system, for example the iron(II) salt, to the batch beforethe polymerization, while hydrogen peroxide is metered in simultaneouslywith the monomers but separately therefrom. Iron(II) salt is usuallyused in concentrations of from 10 to 200 mg/l of Fe⁺⁺ ion, based on thetotal dispersion, higher and lower concentrations also being possible.Hydrogen peroxide (calculated as 100%) is added in amounts of, forexample, from 0.2 to 6.0% by weight, based on the monomer. This amountis in addition to the amount of hydrogen peroxide which is used for thestarch degradation.

In addition to the redox initiators, conventional initiators, such asoil-soluble or only slightly water-soluble organic peroxides or azoinitiators, may be concomitantly used. In particular, the addition offurther reducing agents, which are preferably initially taken with theiron salt before the polymerization, has advantages. Examples ofsuitable reducing agents are sodium sulfite, sodium disulfite, sodiumhydrogen sulfite, sodium dithionite, ascorbic acid and the sodium saltof hydroxymethanesulfonic acid.

The molecular weight of the grafted-on polymer may additionally beestablished by the concomitant use of chain-transfer agents orregulators, such as n-dodecyl mercaptan, tert-dodecyl mercaptan, n-butylmercaptan or tert-butyl mercaptan. Odorless regulators, such asterpinolines, are particularly suitable (cf. EP-A-1191044).

The polymerization is carried out in general at a pH of from 2.5 to 9,preferably in the weakly acidic range at a pH of from 3 to 5.5. The pHcan be brought to the desired value before or during the polymerizationusing conventional acids, such as hydrochloric acid, sulfuric acid oracetic acid, or using bases, such as sodium hydroxide solution,potassium hydroxide solution, ammonia, ammonium carbonate, etc. It ispreferable to bring the pH of the aqueous polymer dispersions to from 5to 7 after the polymerization by adding sodium hydroxide solution,potassium hydroxide solution or ammonia.

The concentration of the novel dispersions is, for example, from 10 to40, preferably from 18 to 40, % by weight. A 25% strength aqueouspolymer dispersion has, for example, a viscosity of from 3 to 300 mPa·s.

The novel dispersions have a very small particle size; for example, itis below 120 nm. The mean particle size of the dispersed polymerparticles is preferably from 50 to 100 nm. The particle size can bedetermined, for example, by laser correlation spectroscopy or byturbidity measurement.

In order to increase the shelf life of the aqueous polymer dispersions,it is advantageous to bind the heavy metal ions used in the redoxsystem, after the polymerization, by adding at least one complexingagent. For example, complexing agents such as ethylenediaminetetraaceticacid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid,polyaspartic acid, iminodisuccinic acid, citric acid or alkali metal orammonium salts thereof are suitable for this purpose. The amount ofcomplexing agents used depends on the amount of heavy metal salt whichis to be complexed. Usually, the complexing agents are used in an amountof from 1 to 10, preferably from 1.1 to 5, mole per mol of heavy metalion.

The novel polymer dispersions are preferably used as surface sizes butcan also be employed as engine sizes. They generally have a weaklyanionic charge character and only a slight tendency to frothing. Theyare suitable for the surface sizing of all paper qualities produced inpractice, for example of alum-containing or alum-free papers, papersfilled with kaolin or chalk and base papers which contain groundwood orwaste paper and can be produced under either acidic or neutral oralkaline conditions and may be unsized or presized in the paper pulp,for example with alkylketene dimer or alkenylsuccinic anhydride.

The novel dispersions can be processed by all methods customary forsurface sizing and can be applied to the surface of paper in the sizepress liquor. Use in aqueous solution together with 5 to 20% by weightof starch and, if required, pigments and optical brighteners in the sizepress or in modern application units, such as a film press, speedsizeror gateroll, is customary.

The amount of size in the liquor depends on the desired degree of sizingof the papers to be finished. Usually, the concentration of the noveldispersions in the liquor is from 0.1 to 2.0% by weight of solidsubstance, preferably from 0.2 to 1.0% by weight. The amount applied tothe paper is determined by the wet absorption of the unsized or presizedpapers. Wet absorption is to be understood as meaning the amount of sizepress liquor which, based on the dry fiber, can be absorbed by thelatter and which can be influenced, inter alia, by the presizing in thepaper pulp. Depending on the wet absorption, the amount of the solventabsorbed by the paper is in most cases from 0.03 to 1.2% by weight ofsolid substance, based on dry fiber, preferably from 0.1 to 0.8% byweight.

The size press liquor may additionally contain finely divided pigmentsfor improving the printability, for example chalk, precipitated calciumcarbonate, kaolin, titanium dioxide, barium sulfate or gypsum.Furthermore, the addition of optical brighteners for increasing thewhiteness, with or without the addition of carriers, e.g. polyethyleneglycol, polyvinyl alcohol or polyvinylpyrrolidone, is customary duringuse on graphic arts papers. The good compatibility of the noveldispersions with optical brighteners is particularly advantageous, sothat papers having high whiteness can be obtained.

Also particularly advantageous is the nonsensitivity of the noveldispersions to the addition of electrolytes, such as sodium, calcium oraluminum ions, which may be present in the size press liquor in manycases, for example through migration from the base paper to beprocessed, or may be deliberately added for increasing the conductivity.

The novel size dispersions are particularly suitable for the productionof graphic arts papers which are used for all conventional modernprinting processes. In inkjet printing, for example, high inkadsorptivity and rapid drying without strike-through are requiredtogether with good ink hold-out, maintenance of high ink density andhigh resolution as well as good smudge resistance and water resistance.In color printing, crisp edges are required, and the individual coloredinks must not run into one another and should have high color strength,brilliance and lightfastness. These requirements can be met in anoutstanding manner by the novel dispersions.

The use of the papers finished with the novel dispersions inelectrophotographic printing processes, such as in laser printers andcopiers, simultaneously requires good toner adhesion, i.e. the tonermust adhere with high smudge resistance to the paper. This requirement,too, can be met in an outstanding manner by the use of the noveldispersions, in particular on papers presized, for example, withalkyldiketene.

In the examples which follow, parts and percentages are by weight,unless otherwise evident from the context.

EXAMPLES Preparation of Novel Polymer Dispersions Example 1

In a polymerization vessel equipped with a stirrer, reflux condenser,metering apparatuses and means for working under a nitrogen atmosphere,9 kg of starch (Amylex 15 Südstärke) and 29.78 kg of water wereinitially taken and were heated to 85° C. in the course of 25 minuteswhile stirring. 0.12 kg of a 25% strength aqueous calcium acetatesolution and 0.226 kg of a 0.015% strength commercial enzyme solution(α-amylase) were then added. After 30 minutes, the enzymatic starchdegradation was stopped by adding 0.6 kg of 100% pure acetic acid. Themolecular weight M_(n) of the degraded starch was 6 000. 0.15 kg of a10% strength aqueous iron(II) sulfate solution was then also added. Thetemperature of the reaction mixture was kept at 85° C. At thistemperature, a mixture of 24.6 kg of styrene, 5.4 kg of 1,3-butadieneand 1.5 of tert-dodecyl mercaptan, 0.038 kg of a 40% strength aqueousemulsifier K 30 solution and 10.1 kg of water were then added in courseof 120 minutes. Separately therefrom, 6 kg of a 15% strength hydrogenperoxide solution were added in the course of 30 minutes, and then 2 kgof a 15% strength hydrogen peroxide solution in the course of 105minutes. 1 kg of a 10% strength aqueous tert-butyl hydroperoxidesolution was then metered in at 60° C. in the course of 60 minutes.Thereafter, 0.225 kg of a 40% strength Trilon B solution and 1.2 kg of a25% strength aqueous sodium hydroxide solution were added to thedispersion and the latter was then physically deodorized for 4 hourswith 10 kg of steam per hour. An aqueous dispersion having a solidscontent of 40%, a light transmittance of 80 and a particle diameter of114 nm was obtained.

Example 2

In a polymerization vessel equipped with a stirrer, reflux condenser,metering apparatuses and means for working under a nitrogen atmosphere,9 kg of starch (Amylex 15 Südstärke) and 29.78 kg of water wereinitially taken and were heated to 85° C. in the course of 25 minuteswhile stirring. 0.12 kg of a 25% strength aqueous calcium acetatesolution and 0.226 kg of a 0.21% strength commercial enzyme solution(α-amylase) were then added. After 30 minutes, the enzymatic starchdegradation was stopped by adding 0.6 kg of 100% pure acetic acid. Themolecular weight M_(n) of the degraded starch was 6 000. 0.15 kg of a10% strength aqueous iron(II) sulfate solution was then also added. Thetemperature of the reaction mixture was kept at 85° C. At thistemperature, a mixture of 24.6 kg of styrene, 5.4 kg of butadiene and0.6 kg of terpinolene, 0.038 kg of a 40% strength aqueous emulsifier K30 solution and 10.1 kg of water were then added in course of 120minutes. Separately therefrom, 6 kg of a 15% strength hydrogen peroxidesolution were added in the course of 30 minutes, and then 2 kg of a 15%strength hydrogen peroxide solution in the course of 105 minutes. 1 kgof a 10% strength aqueous tert-butyl hydroperoxide solution was thenmetered in at 60° C. in the course of 60 minutes. Thereafter, 0.225 kgof a 40% strength Trilon B solution and 1.2 kg of a 25% strength aqueoussodium hydroxide solution were added to the dispersion and the latterwas then physically deodorized for 4 hours with 10 kg of steam per hour.An aqueous dispersion having a solids content of 40%, a lighttransmittance of 94 and a particle diameter of 81 nm was obtained.

Comparative Example 1 (Example 1 of EP-A-0 735 065)

First Process Stage

500 parts of water were initially taken in a stirred double-jacketcontainer having a blade stirrer, reflux condenser and N₂ feed line and126 parts of potato starch acetate ester having a degree of substitutionof 0.03 were added while stirring. Thereafter, 0.3 part of α-amylase LPwas added and the mixture was heated to 80° C. and kept at thistemperature for 2 hours. After the addition of 3 parts of sodiumperoxodisulfate, dissolved in 15 parts of water, a mixture of 30 partsof styrene, 15 parts of n-butyl acrylate and 1 part of acrylic acid wasmetered in continuously over a period of 40 minutes. After the end ofthe feed, stirring was effected for a further 60 minutes at 80° C.

Second Process Stage

1 part of sodium hydroxymethanesulfinate, dissolved in 10 parts ofwater, was added at 80° C. to the dispersion obtained in the firstprocess stage. Immediately thereafter, a mixture of 90 parts of styreneand 45 parts of n-butyl acrylate, a mixture of 16 parts oftrimethylammoniumethyl methacrylate chloride in 14 parts of water and amixture of 3 parts of hydrogen peroxide in 35 parts of water weremetered in continuously over a period of 150 minutes, beginning at thesame time but separately from one another. After the end of themetering, stirring was effected for 20 minutes at 85° C. and, aftercooling, a coagulum-free polymer dispersion having a solids content of33%, a pH of 5.5 and a mean particle size of 110 nm was obtained.

Comparative Example 2 (Example 3 of EP-A-0 735 065)

First Process Stage

500 parts of demineralized water were initially taken in a stirred 1 ldouble-jacket container having a blade stirrer, reflux condenser and N₂feed line and 315 parts of an oxidatively degraded potato starch solubleat elevated temperatures were added while stirring. Thereafter, 0.3 partof α-amylase LP was added and the mixture was heated to 80° C. Thistemperature was maintained for 2 hours and then 3 parts of 37% strengthformaldehyde solution were added. After the addition of a further 3parts of sodium peroxodisulfate, a mixture of 15 parts of styrene, 30parts of n-butyl acrylate and 2 parts of acrylic acid was metered incontinuously in the course of 35 minutes. Polymerization was continuedfor a further hour.

Second Process Stage

1 part of sodium hydroxymethanesulfinate, dissolved in 10 parts ofwater, was added at 82° C. to the dispersion obtained in the firstprocess stage. Immediately thereafter, a solution of 3 parts of hydrogenperoxide in 10 parts of water and a solution of 30 parts oftrimethylammoniumethyl methacrylate chloride in 20 parts of water and amixture of 90 g of styrene and 60 g of n-butyl acrylate were metered incontinuously over a period of 120 minutes, beginning at the same timebut separately from one another. Polymerization was continued for afurther hour at this temperature and, after the addition of 9 parts of20% strength sodium hydroxide solution, a coagulum-free dispersion wasobtained.

Comparative Example 3 (Example 1 )of EP-A-0 257 412)

31.8 g of an oxidatively degraded starch and 219 g of water wereinitially taken in a 1 l four-necked flask equipped with a stirrer,reflux condenser, metering apparatuses and means for working under anitrogen atmosphere and were heated to 85° C. in the course of 30minutes while stirring. 1 g of a 1% strength aqueous calcium acetatesolution and 1.6 g of a 1% strength commercial enzyme solution(α-amylase) were then added. After 20 minutes, the enzymatic starchdegradation was stopped by adding 4 g of glacial acetic acid. Theintrinsic viscosity of the starch after this treatment was 0.21 dl/g. 7g of a 1% strength aqueous iron(II) sulfate solution and 0.34 g of a 30%strength hydrogen peroxide were also added. The temperature of thereaction mixture was kept at 85° C. At this temperature, a mixture of 40g of acrylonitrile and 33.5 g of n-butyl acrylate was then added in thecourse of 1 hour and, separately therefrom, 61 ml of a 0.7% strengthhydrogen peroxide solution were then added, likewise in the course ofone hour. After all the monomers had been metered in, polymerization wascontinued for a further hour at 85° C. A dispersion having a solidscontent of 26.3% was obtained. The light transmittance of the dispersionwas 96%.

Comparative Example 4 (Example 2 of JP-A-58/115-196)

500 parts of a 6.6% strength aqueous solution of an oxidatively degradedpotato starch were initially taken in a 2 l flask provided with astirrer and a reflux condenser. The degraded starch had an intrinsicviscosity η_(i) of 0.27 dl/g and a degree of substitution of 0.034 molof carboxyl group per mole of glucose unit. 44 parts of styrene, 71.7parts of n-butyl acrylate and 21.7 parts of tert-butyl acrylate as wellas 3 parts of potassium peroxodisulfate in 50 parts of water were thenadded to the initially taken mixture heated to 80-90° C. An anionicpolymer dispersion having a solids content of 25% and a lighttransmittance of 90 was obtained.

Comparative Example 5 (Cationic Dispersion 2 of EP-A-0 307 816)

20.7 parts of an 82% strength aqueous cationic potato starch (η_(i)=0.1dl/g, degree of substitution 0.025 mol of nitrogen per mole of glucoseunit) were dissolved in 133 parts by weight of water at 85° C. whilestirring in a polymerization vessel equipped with a stirrer, meteringapparatuses and a means for working under nitrogen. 3.7 parts of glacialacetic acid and 0.03 part of iron sulfate (FeSO₄.7H₂O) were added,followed by 0.8 part of 30% strength hydrogen peroxide and, after 20minutes, 0.8 g of 30% strength hydrogen peroxide. An emulsion of 44parts of n-butyl acrylate and 39 parts of styrene in a solution of 0.045part of sodium laurylsulfate in 29 parts of water and, beginningsimultaneously therewith, from a second feed vessel, 14 parts of a 5.5%strength hydrogen peroxide solution were then metered in the course of 2hours. After the end of the monomer addition and the hydrogen peroxideaddition, the reaction mixture was polymerized for a further hour at 85°C. A cationic dispersion having a solids content of 34% and a lighttransmittance of 86 was obtained.

The aqueous polymer dispersions prepared according to examples 1 and 2and comparative examples 1 to 5 were tested with respect to theirefficiency as surface sizes for paper. In each case, the Cobb valueaccording to DIN 53132 and the ink flotation time according to DIN 53126were determined.

The test paper used was a non-presized paper filled with PCC(precipitated calcium carbonate) and comprising 70% of birch sulfate and30% of pine sulfate. In order to determine the surface sizing effect ofthe aqueous polymer dispersions prepared according to the examples andthe comparative examples, said dispersions were each diluted to apolymer content of 2 g/l and applied with the aid of a size press to thetest paper described above. The test papers were then dried andconditioned and were tested by the methods mentioned above. The valuesdetermined for the ink flotation time and the Cobb value are shown inthe table. The lower the Cobb value and the longer the ink flotationtime, the more efficient is the size:

Size prepared Ink flotation time/ according to Cobb/g/m² min. Example 127 35 Example 2 33 35 Comparative example 1 92 0 Comparative example 2106 0 Comparative example 3 48 7 Comparative example 4 40 12 Comparativeexample 5 55 4

1. An aqueous paper size composition, which comprises: dispersed polymerparticles; at least one degraded starch having a molecular weight Mn offrom 500 to 40,000; at least one water soluble catalyst; and at leastone complexing agent; wherein the dispersed polymer particles comprise acopolymer consisting of (a) from 50 to 99% by weight of styrene and/ormethylstyrene, (b) 1 to 50% by weight of 1,3-butadiene and/or isopreneand (c) from 0 to 40% by weight of other ethylenically unsaturatedcopolymerizable monomers, the sum of the monomers (a), (b) and (c)always being 100%, the copolymer is obtained by free radicalcopolymerization in the presence of from 10 to 40% by weight, based onthe weight of monomers (a), (b) and (c), of the at least one degradedstarch having a molecular weight Mn of from 500 to 40,000, the at leastone water soluble redox catalyst comprises hydrogen peroxide and atleast one heavy metal salt selected from the series consisting ofcerium, manganese and iron(II) salts, and the mean particle size of thedispersed polymer particles is from 50 to 100 nm.
 2. The paper sizecomposition as claimed in claim 1, wherein the copolymer consists of(a)styrene and (b) 1,3-butadiene.
 3. The paper size composition as claimedin claim 1, wherein a solids content is from 10 to 50%.
 4. The papersize composition according to claim 1, wherein the complexing agent isat least one selected from the group consisting ofethylenediaminetetraacetic acid, nitrilotriacetic acid,diethylenetriaminepentaacetic acid, polyasparatic acid, iminodisuccinicacid, citric acid, a alkali metal salt thereof and an ammonium saltthereof.
 5. The paper size composition according to claim 1, wherein anamount of complexing agent is in the range of from 1 to 10 mole per moleof heavy metal ion present.